Sounds like you're doing Yasso 800's (except they're suppose to be closer to 5K pace I think). With the exception of this past season I usually do Yasso 800s on the track in the last six to eight weeks leading up to my taper before a marathon. As Bart Yasso suggests, I try work up to doing 10 of them at the pace I want for my marathon time (10x 800 at 3min, 30sec pace for a 3:30 marathon. I think I'm going to try to work on some 1600m intervals next season.